Needles are frequently used in electromyography (EMG) for detecting electrical signals within the body. Commonly, needles used in electromyography have one or two electrodes, termed monopolar and bipolar, respectively.
If the diameter of a needle is too large, it can cause significant pain to the person in whom it is inserted. Thus it is desirable to have as small a diameter as possible.
For needles having bipolar electrodes, one of the electrodes is usually a reference conductor while the other is used as an active conductor. A common or ground conductor is placed on the skin externally of the point at which the body's electrical activity is being monitored. This placement of the common conductor on the skin takes time during the EMG set up procedure. Further, the signal quality achieved with a common electrode remote from the measurement point of the active and reference electrodes is sub-optimal.
Some needles use a beveled tip. That is, the needle tip is cut at an angle from one side to the other, presenting a substantially elliptical face in the plane of the cut (assuming that the needle is of circular cross-section). Because of the elliptical shapes of the electrodes exposed on the elliptical face, the distance between the central electrode and the outer concentric electrode is not uniform across the exposed elliptical face of the needle at its tip. This lack of uniformity of distance between the two electrodes can lead to inaccuracies in the signal detection.
A further problem with existing beveled tip concentric needles is that when they are constructed with a small outer diameter, the gap between the active and reference electrodes is often quite small, leading to lower amplitude signals and smaller effective measurement area. There is also a tendency for the electrodes to short circuit after the tip becomes worn by repeated insertions. Also, the area of the exposed surface of the inner conductor is small, resulting in low signal amplitudes.